The invention relates to a device and a method for constructing a layer body from a plurality of superimposed layers of free-flowing material, in particular particulate material, on a build platform within a build space, the layers being solidified and joined together in locally predetermined areas by the action of a binder so that at least one molding is formed by the solidified and joined areas of the layers, the device comprising a discharging device which is movable back and forth over the build space in at least one discharge direction and which has at least one discharge opening from which the free-flowing material is dischargeable in individual superimposed layers during the movement of the discharging device.
A computer-controlled method for producing three-dimensional moldings is described in EP 0 431 924 B1. Free-flowing particulate material is applied in a thin layer to a build space which is surrounded, as appropriate, by container walls and applied to a build platform, and a binder is selectively printed thereon, using a print head, according to computer data. The particle area onto which the binder is printed sticks together and solidifies under the influence of the binder and, if necessary, an additional hardener. The build platform is then lowered by a distance of one layer thickness into a build cylinder and provided with a new layer of particulate material, which is also printed as described above.
These steps are repeated until the desired height of the molding is achieved. A three-dimensional object is thus produced from the printed and solidified areas.
After it is completed, the molding produced from solidified particulate material is embedded in loose particulate material and is subsequently removed therefrom. This is done, for example, using an extractor. This leaves the desired molding, from which the remaining adhering particles are removed, for example by brushing.
Other powder-supported rapid prototyping processes work in an identical or similar manner, for example, selective laser sintering or electron beam sintering, in which a loose particulate material is also applied in layers and selectively solidified with the aid of a controlled physical radiation source. The aforementioned method is summarized under the term, “three-dimensional printing” or “3D printing.”
However, the provision of a build container or build cylinder having a build platform which may be lowered vertically into the build container requires a high degree of technical complexity for sealing the build container wall against the build platform to prevent uncontrolled outflow of the particulate material through the gap between the build platform and the build container wall. Another disadvantage of a lowerable build platform is the constantly increasing weight to be moved on the build platform as the building process progresses. In particular during application of another layer, it may be necessary to lower the build platform by a distance of more than one layer thickness and then to raise it again to the dimension required in order to adjust the layer thickness with sufficient accuracy.
In a reversing operation of this type, not only does the entire weight of the power feedstock, including the build platform, need to be overcome but also the friction forces between the power bed and the build container wall. This results in high stresses on the guides and drives of a vertically moving build platform, in particular in the case of large build spaces and high feedstock densities.
In contrast, EP 1 872 928 A1 proposes to raise the discharging device and the print head relative to the build platform instead of lowering the build platform relative to the discharging device and print head for building larger three-dimensional moldings as the layering process progresses. This publication furthermore proposes to construct solid walls made of particulate material by solidifying the edge areas of the applied particulate material and by forming, from these walls, a build space-delimiting build container in whose inner chamber moldings of a selective size and shape may be constructed. It is alternatively proposed to construct moldings on the build space in a free-standing manner without using a build container for encompassing and supporting previously applied layers. For this purpose, the discharging device is designed as a dosing device which may undergo controlled activation and deactivation for the controlled output of a predetermined, preferably constant, linear volume flow of particulate material per length unit and per time unit, so that particulate material is not unnecessarily strewn around the molding to be built or is not “emptied” prematurely and thus does not lose its function while the layer is being deposited. However, such dosing-controlled and “intelligent” discharging devices have a relatively complex structure and are correspondingly expensive.
“Unintelligent” discharging devices of a simpler design, on the other hand, are unable to selectively dose particulate material or are not switchable. For example, they include a scraper moving in the discharge direction or a counter-rotating roller or an oscillating blade.
These devices then distribute a quantity of material in front of the discharging device on the build space in the discharge direction. The amount of particulate material then has to be sufficiently measured before the discharging device travels over the build space.
Other design shapes may guide a quantity of material in two directions around the surface to be coated. These include discharging devices which comprise a simple, elongated hopper which has a slot on the underside as the discharge opening for discharging particulate material. In another embodiment, at least one of the two hopper walls, for example, is replaced by a counter-rotating roller.
In order for the process to progress as described, the discharging device must pass completely over the area to be coated. However, a discharging device according to the aforementioned, simple and “unintelligent” design, loses a remaining quantity of material in front of the discharging device in the discharge direction once the edge of the build space has been reached. This quantity of material would then be unavailable for the remaining building process. Nevertheless, it would be desirable to return this lost material to the discharging device for further layering.
A possibility for largely avoiding lost material is known, for example, from US 2005/0280185 A. In this publication, the quality of particulate material in the discharging device is predetermined by a sensor system.
The discharging device carries along a predetermined material quantity which is sufficient for coating the desired surface without producing too many waste particles after passing over the edge of the build space. In this case, however, the quantity must be very precisely determined to avoid too small a dosing in each cases, which would result in insufficient layering. The continuous decrease in the quantity of material in the discharging device during travel has proven to be another disadvantage of this method, resulting in an unsteady coating process. This may cause the feedstock density to be greater at the beginning of discharging device travel than at the end of the travel, due to the greater weight of the material, when a residual amount is left over in the discharging device.
In contrast, the object of the invention is to refine a method and a device of the aforementioned type in such a way that a variable and coating material-saving adjustment of the build space size is possible while simultaneously maintaining high coating quality, despite a simple and cost-effective design of the discharging device.